The greater bandwidth and low transmission loss of optical fibers coupled with the high speed capabilities of optical devices has led to the evolution of high data rate transmission systems in the telecommunications industry. Optically based transmitters and receivers have enabled transceiver modules to evolve in a variety of forms, all to accomplish two directional communications at optical frequencies with simultaneous transmission and reception (full-duplex) or at sequential transmission and reception (half-duplex).
By and large, transceiver based systems utilize two optical fibers to accomplish this goal, one to transmit light signals from the transmitter side of the module and a second fiber to acquire the optical signal for the receiver side. More recently, interest has centered on transceiver designs which utilize a single optical fiber for both the transmission and reception of the optical signal. Modules which utilize this single fiber approach we refer to here as bi-directional modules. In general, these can be of two subtypes, systems which transmit and receive optical radiation at the same wavelength, and systems which utilize two different wavelengths, one for transmission in one direction and a second wavelength for transmission in the opposite direction. These are referred to as single wavelength bi-directional and dual wavelength bi-directional, respectively. The present disclosure refers to the latter type of configuration.
The conventional art in the optical design of dual wavelength bi-directional modules typically utilizes a semiconductor source such as an LED or injection laser, a semiconductor photodetector such as a PIN photodetector or APD, a beam splitting component, and additional filters and focusing optical elements. A conventional dual wavelength bi-directional module is as disclosed in U.S. Pat. No. 5,127,075 to Althaus et al, the disclosure of which is specifically incorporated herein by reference. The beam splitting component is utilized to spatially separate the incoming and outgoing light signals, directing the outgoing transmission signal from the source onto the fiber and directing the incoming signal from the fiber to the detector. Dichroic filters can be used to prevent source radiation from reaching the detector and to prevent incoming signals from the fiber from reaching the source. In some designs, the filter functions are built into the beam splitting element. Focusing elements can be used to maximize the optical coupling between the source and the fiber and the fiber and the detector. The present disclosure proposes an in-line configuration of the detector and source, which is enabled by a special design of the photodetector component utilized in the assembly.
The use of photodetectors has been rather widespread, and with the emerging potential for fiber to the home (FTTH) applications, there is a demand for increased application. One such application is for use in a transceiving mode, in particular a bi-directional link. While some bi-directional links are in the form of packaged devices in a housing orthogonal to one another having wavelength selective optics to effect the transmission of one wavelength from an LED or laser, and reception of another wavelength into a detector, these configurations are often undesirable. The drawback for many applications is the need to effect alignment of the fiber, devices and optics, creating a labor intensive and therefore a higher cost product. Furthermore, these devices require beam splitting and/or filtering elements to assure that signals are properly directed from and to the respective transmitter and detector devices. The push for the FTTH application requires a low cost product.
The basic performance of a PIN photodetector is described presently. Semiconductor pn junctions are employed widely for photodetection. The basic physics of their use in this application is as follows. Turning to FIG. 1, we see the energy band diagram of a pn junction used as a photodetector. Light absorbed at the p region of the junction, creating an electron-hole pair as shown. If the absorption of the light occurs at a point of the p-side that is within a diffusion length (the average length that a minority carrier will diffuse before recombining with an opposite carrier) of the depletion region edge, the electron has a high probability of being collected and will drift across the depletion region. Such an electron will then contribute a charge e to the external circuit, thereby giving an electrical indication of the optical signal absorbed by the junction photodiode. Should the light be within the absorption band of the detector and be received on the n-side of the depletion region of the junction, another electron-hole pair will be created, and the hole will traverse to the junction again by diffusion, and then drift across the junction. Again, this will result in a charge flow e across the external load. Alternatively, and preferably, the photon could be absorbed within the depletion region, creating an electron-hole pair. The electron and hole created will drift in opposite directions under the field of the bias potential. In this arrangement, each carrier will traverse a length that is less than the depletion width and the contribution to the charge flow in an external circuit is e as determined from basic transport equations. This method is most desirable, since each absorption gives rise to a charge of magnitude e, and the delay in current response time due to finite diffusion time is minimized. This mode of operation is effected by fabricating a structure having a layer of intrinsic (i) semiconductor sandwiched between the p and n layer, and is conventionally referred to as a PIN diode. In practice this layer is unintentionally doped due to the uncontrollable background impurities, but typically is characterized by a background n-type carrier concentration of 3.times.10.sup.15 /cm.sup.3 or less. For the purposes of discussion of the invention of the present application, intrinsic is understood to be the above definition. The intrinsic layer is a high resistivity layer and the potential drop of the bias potential is greatest across the intrinsic layer. Furthermore, the intrinsic layer is generally made thick enough to assure that most incident photons are absorbed within this layer.
In FIG. 2, we see a cross sectional view of a conventional PIN photodetector for application in the near IR region between 1.0-1.65 microns. The intrinsic absorption layer consists of In.sub.x Ga.sub.1-x As ternary material which is epitaxially grown lattice matched on a semiconductor substrate. The substrate is generally chosen to be transparent in the wavelength range desired to be detected, and in the case of an In.sub.x Ga.sub.1-x As absorption layer, an n.sup.+ InP substrate is chosen as it is transparent in the range 1.0-1.65 microns in wavelength. Under operating conditions, the intrinsic layer is depleted fully by a top pn junction. The PIN structure can be achieved by simply growing a top layer of p.sup.+ In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y or InP over a layer of intrinsic In.sub.x Ga.sub.1-x As (not shown), but in most practical devices, fabrication is effected by having a localized p.sup.+ region 201 formed by diffusion of a suitable dopant, for example Zn, into the cap layer of InP or In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y through a suitable mask, for example SiO.sub.2 deposited by conventional techniques on the top layer. The desired effect of this practical technique of fabrication is a planar structure, with a well defined junction area (by virtue of the mask diffusion technique) and minimum surface current leakage by virtue of the buried junction. A PIN photodetector of this structure can be illuminated either from the top through the pn junction or from the rear through the transparent InP substrate if a suitable opening in the n-side metallization is provided.
The device operates under the condition of reverse bias to effect the desired field direction to facilitate carrier flow upon absorption of light of the proper wavelength. The reverse bias potential of a few volts is usually enough to fully deplete the intrinsic layer of carriers, and in the absence of light signals, only a small reverse current flows across the boundaries. Finally, it is important to recognize that due to the absence of a gain mechanism in the PIN diode, the gain-bandwidth product is nearly equal to the bandwidth itself, the bandwidth determined by the transit time of electron-hole pairs, and accordingly by the thickness of the intrinsic layer. Accordingly, the thickness of the intrinsic layer's effect on absorption efficiency must be balanced against the effect on time of transit. In practice, the bandwidth of the PIN detector is limited by factors such as the RC time constant of the packaged device, and bandwidths on the order of several GHz are achievable.
One way to effect a more simple, and thereby lower cost product is to fabricate an in-line bi-directional link. In U.S. Pat. No. 4,577,209 to Forrest, et al., the disclosure of which is specifically incorporated herein by reference, a bi-directional link is disclosed that is a PIN detector having a bored center section for the insertion of an optical focusing element fiber. An LED or other source is in communication with the fiber through the hole in the PIN detector, and thus a bi-directional link is effected. The link disclosed in this reference is based on a single wavelength, and attempts to reduce the detrimental effects of radiation traveling in one direction which can be reflected from light-reflecting interfaces in the system. This will result in signal interference and cross-talk. U.S. Pat. No. 4,709,413 which is related to the '209 reference and the disclosure of which is also specifically incorporated herein by reference, like the '209 reference attempts to provide signal or channel isolation in single wavelength bi-directional links.
Both of these references require a hole etched in the detector through to a light source, the hole in one embodiment having a coupling member, such as a fiber disposed therein, and an active area of a detector that is designed to capture all of the radiation incident thereon from a fiber. The systems of this reference attempt to effect a bi-directional link that provides good isolation between the transmitter and receiver. However, the systems disclosed in these references are not designed to effect communication using multiple wavelengths and cannot achieve the degree of isolation that is effected in a multiple wavelength system. These systems require the alignment of the optical fiber to the active area of the device to maximize the incident radiation on the active area of the detector, as well as alignment of the source to a coupling element or the opening of the fiber. To reiterate, the inventions of the referenced patents do not achieve the isolation of the signals and require labor intensive alignment of the various components of the system. What is needed is a system that will effect isolation between the transmitted and received signals, preferably by the use of one wavelength of light for signal transmission, and one for signal reception in an in-line system. Furthermore, what is needed is a system that does not require a great deal of optical coupling elements, and thus optical alignment or complex fabrication techniques.